pull: Make '--dry-run' behave as expected.

* guix/scripts/pull.scm (show-help): Document '--dry-run'.
(build-and-install): Add #:dry-run? parameter and honor it.
(guix-pull): Remove (assoc-ref opts 'dry-run?) condition.  Instead, pass
it as #:dry-run? to 'build-and-install'.
* doc/guix.texi (Invoking guix pull): Document '--dry-run'.  Move
'--verbose' to the bottom.